1. Field of the Invention
The present invention pertains to water-in-oil emulsions wherein the oil phase is a self-emulsifying organopolysiloxane. The water-in-oil emulsions can be prepared in the absence of emulsifying surfactants.
2. Description of the Related Art
Emulsions of organopolysiloxanes are known, and are of two major types. The first, and most common type, are oil-in-water (“O/W”) emulsions. Such emulsions are produced by dispersing an organopolysiloxane (“silicone”) into water with the aid of a surfactant or dispersing aid. A large number of different types of surfactants, or “emulsifiers” may be used, including anionic surfactants, cationic surfactants, zwitterionic surfactants, and non-ionic surfactants, particularly the latter. Partially hydrophobic inorganic particles in the nanometer size range may also be used, these particles containing both hydrophobic and hydrophilic portions which mimic conventional surfactant structure.
Less common are water-in-oil (W/O) emulsions. Preparation of such emulsions has been described in the literature as technically difficult, requiring different surfactants and surfactant concentrations as compared to O/W emulsions, and possessing different stability profiles. In the case of both types of emulsions, freedom from separation, i.e. storage stability, is a major concern.
A further problem with both types of emulsions is that a considerable surfactant quantity is necessary to produce small dispersed phase particle sizes and maintain stability. Unfortunately, in many applications, particularly in the personal care sector, e.g. cosmetics, body lotions, shampoos, etc., the choice of surfactant is limited due to potential irritation. Furthermore, many of the surfactants can leave the skin, for example, with a soapy or tacky feel.
Stable W/O emulsions have been described in the literature as difficult to make compared to O/W type because of their inherent instability. One of the most common reasons for their inherent instability is the absence of clearly defined electrical double layer surrounding the dispersed phase droplets, which would prevent the droplets from coalescence. In almost all examples previously reported, a surfactant or a solubilizer as well as high shear is essential to make a stable W/O emulsion.
Water-in-oil (W/O) emulsions, also called invert emulsions, are highly sought after for applications in personal care, surface care, pharmaceuticals and in the oil and gas industry. However, W/O type emulsions are more difficult to make compared to O/W type emulsions because of their inherent instability (Klein, K. Formulating Water-in-Oil Emulsions: A Scary Endeavor, Cosmetics & Toiletries, 24-25, (2006). See Also, makingcosmetics.com/articles/27-how-to-make-water-in-oil emulsions). To make a water-in-oil emulsion, an external surfactant or solubilizer is almost always necessary to make the oil phase and the aqueous phase blend with each other. U.S. Pat. Nos. 5,853,711 and 7,914,772 B2 describe W/O compositions for cosmetic application. U.S. patent application Ser. No. 11/699,906 describes the use of a silicone W/O emulsion composition for drug delivery. Patent application PCT/US2004/040603 describes a water-in-oil emulsion wax composition for cleaning and polishing. International patent application WO 2000069423 Al describes invert emulsions for cleaning skin or hard surface by application with a wipe. The use of external surfactants, which mostly are of low molecular weight, can sometimes negatively impact the properties of the composition. For example, in cosmetics many surfactants are of concern for their irritancy and allergic reactions on the skin. For paint formulation, leaching of surfactants has a detrimental effect on the appearance of the painted surface. Besides, the addition of emulsifiers adds complexities to the system and adds to processing steps and cost. Thus, it would be advantageous if W/O emulsions of silicones can be made without the addition of any external surfactants.
U.S. Pat. No. 5,643,555 describes emulsifier-free W/O emulsions for cosmetics applications, which were stabilized by polyalkylsilsesquioxanes. U.S. Pat. No. 6,379,680 B2 describes emulsifier-free W/O emulsions stabilized by finely dispersed, micronized inorganic pigments. Also, U.S. Pat. No. 6,391,321 B1 describes both o/w and w/o type emulsifier-free emulsions stabilized by microfine solid particles. These prior arts use the concept of “pickering emulsions” that need finely divided solid particles to stabilize the emulsions. Such stabilizing particles are also included within the definition of “surfactant” herein, as they function in the same manner, albeit being of substantially inorganic nature. Such “pickering” particles may also be termed “dispersants.”
Water-in-oil emulsions have some unique advantages, including the ability to be formulated with additional non-polar ingredients. For example, fragrances and emollients such as lanolin can be added directly by simple mixing, which is not the case with O/W emulsions.
It should be desirable to provide invert emulsions, and particularly microemulsions, which possess the desirable attributes of such emulsions, which are stable, and which do not require the use of surfactants with their disadvantages.